U.S. Pat. No. 5,466,326 discloses a heat-sealing machine, in which the hot air temperature is adjusted during a change in the velocity of feed of the material being welded such that to achieve uniform quality of welding, the hot air temperature is correspondingly increased when the velocity of feed increases and it is correspondingly reduced when the velocity of feed decreases. The adjustment of the hot air temperature is achieved either by interfering with the energy supply to the air heater or, at constant energy supply, by adjusting a deflecting plate, which makes possible a variable mixing of hot and cold air. This machine was designed for carrying out a very specific work process, in which the same materials are obviously processed at all times, since there is, e.g., no reference to a variable pressing device or a pressing device adjustable to different values.